The ContentUpdating event is raised only if the data in a custom XML part is modified when the document is open. This event is not raised when the document is opened and the data in a custom XML part was modified when the document was closed.

void plainTextContentControl1_StoreUpdating(object sender,
Microsoft.Office.Tools.Word.ContentControlStoreUpdatingEventArgs e)
{
MessageBox.Show("The control was changed to the following value: \n\n" +
e.Content + "\n\nThis value is about to be written to the node that is bound to this control.");
}
void plainTextContentControl1_ContentUpdating(object sender,
Microsoft.Office.Tools.Word.ContentControlContentUpdatingEventArgs e)
{
MessageBox.Show("The node that is bound to this control was changed to the following value: \n\n" +
e.Content + "\n\nThis value is about to be written to this control.");
}